In general, in order to reduce access time of an optical disk apparatus, it is important to reduce the size and weight of optical heads. In conventional systems, a track signal, a focus signal, and an RF (high frequency) signal are detected by individual optical systems which require a plurality of optical parts with a correspondingly complex structure. The complicated structure both increases the weight and complexity of the optical head. Attempts have been made to reduce the size and weight of optical pick-ups by utilizing high-density gratings; however, with high-density gratings, the angle of diffracted light tends to deviate due to fluctuations in the wavelength of laser light emanating from the semiconductor laser and must be accounted for in the design. Reduction in cost is also desirable.
It is a current practice to use a Wollaston or Rochon polarizing prism in M-O (magneto-optical) optical heads that split incident beams into two component beams exiting from the prism at small angles (a few degrees) with respect to one another. Moreover, these prisms readily separate the incident beam into TE and TM components. While these features of Wollaston and Rochon prisms do facilitate a single detector package design and are desirable in MO-differential RF signal detection, these prisms are very expensive. In addition, these prisms are relatively heavy.
An approach to this problem is to utilize an optical head which employs a dual diffraction grating for separating light reflected from the optical disk to perform the light signal detecting function. Exemplary of this approach is the disclosure of U.S. Pat. No. 5,029,154 to Sumi et al., in which a quartz substrate has diffraction gratings etched on opposite sides thereof. Sumi et al. is in contrast to approaches such as that of U.S. Pat. Nos. 4,945,525 and 4,672,187 to Yamamoto et al. and Fujita et al., respectively, in which two separate diffraction gratings in spaced relation to one another are used to separate a beam of coherent light into first and second components, such as TE and TM components, for use in generating tracking error, focusing error, and information signals.
While U.S. Pat. No. 5,029,154 to Sumi et al. discloses the concept of a dual diffraction grating, the light beams are separated utilizing gratings of different pitch or different geometrical configuration. Moreover, in Sumi et al., the gratings are on opposite sides of a single substrate. Once one side of a substrate has been etched to provide a first grating, it is extremely difficult to protect the first grating while fabricating the second grating. Moreover, with the approach of Sumi et al., it is necessary to insert a focusing lens between the double grating and the beam reflected from the optical disk.
In view of the Sumi et al. structure and the difficulties associated therewith, there is a need for improvement in and alternative approaches to the dual diffraction grating concept.